Mycelial fungi grown under controlled conditions in a fermenter are a source of protein and therefore useful as a foodstuff. Non-toxic fungal mycelium, particularly suitable as a foodstuff for humans, are disclosed in United Kingdom Pat. Specs. Nos. 1,331,472 and 1,346,062. The preferred non-toxic fungal mycelium is Fusarium graminearum Schwabe I.M.I. No. 145,425. Mechanically agitated aerobic fermenters are suitable for the growing of the biomass and currently known fermenters having mechanical agitation either have a single impeller or a series of impellers mounted on a single shaft for circulating the mass within the fermenter. The fermenter shell is normally cylindrical in shape with the axis of the cylinder vertical and if more than one impeller is provided, they are disposed along the axis of the fermenter. It has been previously proposed to provide two such impellers, one towards the top of the fermenter and one towards the bottom although it is well known that two radial flow impellers will set up two separate regions around each impeller with very little mixing between these regions.
A fermenter is designed to supply a suiable environment for the culture and nourishment of the micro-organism being grown. In the prior art, cultures of yeast and bacteria have been grown aerobically in fermenters and such cultures generally have a low viscosity of the order of water. In comparison, a filamentous mycelial mass such as the preferred micro-organism mentioned above has a much higher viscosity, and is usually non-newtonian. This high viscosity has an adverse effect on the mixing in the fermenter which is necessary for the fungal mycelia to experience a uniform environment. In the case of our preferred micro-organism, 1 gram of mycelia is produced for every 2 grams of glucose, 0.78 grams of oxygen and 0.1 gram of ammonia consumed. Unlike glucose and to a certain extent ammonia which have high solubility in aqueous systems, oxygen has a very low solubility. For example, at 30.degree. C. and at one atmosphere pressure, oxygen has an equilibrium solubility of 0.007 to 0.008 grams per liter depending on the concentration of salts. If at any instant, the system is saturated with oxygen in equilibrium with air, a micro-organism obeying the above stoichiometry at a vessel productivity of 3 grams per liter per hour, will utilize all the dissolved oxygen in 10.8 seconds. This time can be increased to 25.5 seconds using air by increasing the pressure to 20 p.s.i.g. An equivalent increase in this time can be achieved at atmospheric pressure by supplying oxygen enriched air, which has a partial pressure of oxygen at atmospheric pressure, equal to the partial pressure of the oxygen in air at 20 p.s.i.g. Prior art fermenters have not proved to be entirely satisfactory for maintaining adequate dissolved oxygen in this period.